This invention relates to fasteners and is more specifically related to internally threaded fasteners having self locking patches of thermoplastic material adhered to at least a portion of the threads thereof.
Self-locking fasteners of the type in which the self-locking characteristic is derived from a coating such as a patch material adhered to all or a portion of the thread defining surface of the fasteners have proven to be very popular for a wide variety of applications, in order to prevent loosening of the fastener due to vibration and the like in various applications. The prior art discloses various methods and apparatus for applying locking patches of resilient resin or thermoplastic type material to threaded articles. Many of these teachings, however, have proven to be effective only for coating externally threaded fasteners with patches of resilient material and do not find application when it comes to applying such patches on internally threaded fasteners to make them self-locking. To attempt to utilize many of these continuous powder feed type systems with internally threaded fasteners would result in the unwanted coating of surfaces other than the threaded portion of such fasteners.
The problem of providing a coating that will act as a self-locking element to the threads of an internally threaded article such as a nut has presented many difficulties. Such an article may be a friction nut capable of producing a fluid tight seal or the like. A suitable coating for this purpose may be any of numerous resins, such as nylon or other thermoplastic resins. Prior art references that have addressed the problem of forming self locking patches on internally threaded fasteners, have frequently required heating of the fastener to at least about 450.degree. F., prior to directing the thermoplastic material towards the threads to form the self-locking patch.
To insure proper adhesion of the thermoplastic material to the threads with these types of processes, it was also necessary to include an epoxy resin component in the material. Since the entire fastener was preheated to a temperature significantly above the melting point of the thermoplastic resin, great care was required to be used in order to insure that the thermoplastic resin came into contact only with the precise portion of the internal threads of the fastener that was desired to be covered and nowhere else. Otherwise, upon application the resin would tend to adhere to other surfaces of the fastener, causing cosmetic problems, overspray and also improper and inconsistent torque values for the finished self-locking product.
To overcome this problem, various, rather complicated devices were developed. Some devices interrupted the stream of thermoplastic material at regular intervals or indexed the flow of material to coincide with a succession of fasteners passing by the powder supply station. Other systems have utilized special nozzles that were at least partially inserted into the openings of the internally threaded fasteners on a reciprocating basis, to attempt to more precisely control the deposition of powder onto only the desired number of threads and about a desired circumference. Many of these prior art devices also required a large number of powder supply tubes or nozzles in order to provide a separate nozzle for each internally threaded fastener to be coated, such as taught in U.S. Pat. No. 4,366,190. This became particularly troublesome, considering that these systems traditionally required the powder to be entrained in an airstream in order to be directed towards the fasteners.
An additional drawback to such methods and apparatus was presented as a result of the use by these systems of a recirculating powder feed system. Whatever excess thermoplastic powder was directed towards the internal threads of the fastener that did not adhere thereto was frequently, entirely or partially melted as it passed by the highly heated fastener. This resulted in particles fusing together and being recirculated into a powder feed system and then potentially agglomerating even further with other particles. This caused an uneven powder flow of particles of different size, diameter and character to be directed towards subsequent heated fasteners. This resultant variation, combined with pulsing of the airstream entrained powder flow, due to agglomeration of the powder from moisture or the presence of other contaminants, lead to significant variations in torque values of the self locking fasteners and also a shortened useful life of the powder that was recirculated.
Certain disadvantages have also been experienced with other known methods and apparatus to form resilient thermoplastic patches on internally threaded fasteners that do not require the fasteners to be heated prior to application of the patch material. For example, U.S. Pat. No. 4,262,038 discloses a method of producing coated internal threads of a fastener that is only capable of producing a 360.degree. coating and necessitates completely filling the internally threaded opening of the article throughout the complete 360.degree. circumferential extent thereof between the ends, with the thermoplastic resin prior to heating the fastener. This method is slow since it requires the entire cavity of the internally threaded fastener to be filled with powder, even though the vast majority of the powder is not utilized in the coating. Also, this method does not provide for formation of a patch that would either be less than 360.degree. in circumference, or cover fewer than all of the threads of the fastener.
Other prior art systems that do not heat internally threaded fasteners prior to the deposition of the resilient powder present other shortcomings. The system taught in U.S. Pat. No. 3,830,902, requires each fastener to be coated to be placed upon a pin which masks a greater portion of the thread defining surface and establishes a cavity which permits the deposit of plastic powder upon a limited portion of the threads, resulting in the establishment of a plastic patch of limited axial and circumferential extent. These pins require a certain amount of spacing between each successive pin in order to properly position and remove the nuts. It was found that the use of pins upon which fasteners are seated during establishment of plastic patches on the fasteners in such systems was problematic. The pins would wear allowing uncontrolled distribution of powder upon the thread defining surface, even to the extent that the desired clear lead-on thread had not been preserved.
In addition, the limited circumferential extent of a plastic patch produced by such systems provides only a concomitant limited area of adherence between the patch and the threads of the fastener. Thus, where the presence of foreign matter such as water or oil at the interface between the patch and thread defining surface tends to come between the patch and the area of the surface to which the patch adhered, total adherence is diminished sometimes to an unacceptable level. These systems also required use of a powder distribution means that had a continuous flow that had to be indexed with the fasteners travelling thereunder to provide for powder flow only when the threaded surface of the fastener passes below the powder distribution means.
While the prior art systems, referred to above, have proven to be at least somewhat successful in achieving the objects for which they were intended, it has become desirable to have an improved method and apparatus which offers equal or superior speed and quality over existing systems for applying resilient self locking patches to internally threaded fasteners that does not require preheating of the fasteners prior to application of self locking materials, indexing of the fasteners, indexing or interruption of the powder stream to the fasteners, multiple, intricate or reciprocating nozzles for powder deposition, an airstream to be combined with the powder delivery system or the use of powders that have a resin included therein.
While the present invention will be described particularly with respect to applying heat softenable thermoplastic particles to the threads of internally threaded articles, it is to be understood that apparatus and process of the present invention can be used to apply a variety of materials, including resins and resin compounds and pure nylon.
It is therefore an object of the present invention to provide an improved method and apparatus for the manufacture of self locking internally threaded elements wherein the self locking feature is obtained through a thermoplastic deposited onto a selected portion of the internal threaded surface of the element.
Another object of the present invention is to provide an improved method and apparatus for the manufacture of self locking internally threaded elements wherein improved control of the application of the locking body of thermoplastic and thermoplastic application is obtained over a desired arcuate and vertical area of the internal threads of the element and preheating of the fastener is not required.
Yet another object of the present invention is to provide an improved method and apparatus for the manufacture of self locking internally threaded elements wherein the powder flow through the output of the powder delivery system to the elements is continuous and uninterrupted.
Still another object of the present invention is to provide an improved method and apparatus for the manufacture of self locking internally threaded elements that achieves substantially equal results in terms of locking ability regardless of whether the powder used is a resin or has an epoxy constituent.
A further object of the present invention is to provide a method and apparatus for the manufacture of self locking fasteners that allows for greater reusability and more economical use of coating powder.
A still further object of the present invention is to provide a method and apparatus for the manufacture of internally threaded self locking fasteners that utilizes a continuously moving conveyor belt with the internally threaded fasteners delivered onto the belt such that one of the external faces of the nut is in substantially complete contact with the upper surface of the conveyor belt and a portion of one of the sides of the nut is also supported.